This invention relates to an improved magnetic recording medium. More particularly, this invention relates to a magnetic recording medium excellent in magnetic properties and durability, which medium is produced from a magnetic paint improved in the dispersibility of magnetic powders and in the adhesion between the magnetic powders and a binder.
The magnetic powders in a magnetic recording medium, such as magnetic tape or magnetic disc, must have good dispersibility.
A magnetic recording medium is generally produced by uniformly coating a substrate, such as plastic film or sheet, with a magnetic paint prepared by mixing magnetic powders with a binder, various additives and a solvent, drying the painted film and specular surface finish treating the magnetic layer. However, a uniform smooth magnetic layer having a high packing density cannot be obtained, unless agglomerated particles are previously removed by fully dispersing and peptizing the magnetic powders in the magnetic paint.
Nonuniformity in a magnetic layer due to agglomerated particles of magnetic powders exerts a serious adverse effect on the electromagnetic transduction characteristics and magnetic properties of magnetic tape. That is, it causes a lowered output, increased noise, dropout, etc. Further, when the adhesion between magnetic powders and binder is not sufficient, peeling of the magnetic powders from the magnetic film layer is apt to occur while the magnetic recording medium is sliding on a recording head or a reproducing head, and is apt to roughen, with a consequent decrease in the durability of the magnetic recording medium. In the case of conventional magnetic recording media, it has been customary that they are put into practical use without having sufficient durability.
The dispersibility of magnetic powder in a magnetic paint and the adhesion between the magnetic powder and a binder are very important in improving the performance of a magnetic recording medium.
Concerning improvement in the dispersibility of magnetic powder, studies have been made from various viewpoints, and it is proposed to improve the dispersibility by adding a suitable surfactant to a magnetic paint formulation. As such a surfactant, an amine compound and its derivative, a phosphate ester, a polyoxyethylene phosphate or the like is used in most cases, as shown in, for example, a process in which an alkylimidazoline compound is used (Japanese Patent Laid-Open No. 32304/1979), a process in which an alkylpolyoxyethylene phosphate is used after neutralization with an alkylamine (Japanese Patent Laid-Open No. 78810/1978), and a process in which a long-chain alkyl phosphate is used (Japanese Patent Laid-Open No. 147507/1979, Japanese Patent Application No. 49629/1978). Moreover, there are proposals that an improved dispersibility can be obtained by preparing a magnetic paint after treating the surface of magnetic powders, as exemplified by a process in which an alkylpolyoxyethylene phosphate is used (Japanese Patent Laid-Open Nos. 94308/1979 and 49769/1981), and a process in which a titanium coupling agent is used as a surface treating agent for metal powders to attain dispersion stability of a magnetic paint and to prevent the deterioration of magnetic film layer. (Japanese Patent Laid-Open No. 88471/1981). On the other hand, processes in which improvement in durability of magnetic film is attained by improving adhesion between magnetic powders and a binder, include a process in which a silane coupling agent having functional groups reactive with a binder is used (Japanese Patent Laid-Open No. 7310/1979), a process in which magnetic powders are coated with a reaction product of an aminofunctional silane coupling agent and isocyanate compound or an epoxy compound (Japanese Patent Laid-Open No. 143533/1981), a process in which magnetic powder is coated with a titanium coupling agent having an unsaturated bond radicalpolymerizable with a double bond of a binder (Japanese Patent Laid-Open No. 111129/1981), and a process in which a compound having at least four radiation-sensitive acrylic double bonds in the molecule is used as a binder (Japanese Patent Laid-Open No. 40744/1982). In spite of a number of these studies concerning improvement in dispersibility of magnetic powders in a magnetic paint and in durability of magnetic paint film, a satisfactory stage of improvement has not yet been reached.
As a result of an ardent study made under these circumstances to obtain a magnetic recording medium excellent in dispersibility of magnetic powder and durability of a paint layer, the inventors of this invention have found that a magnetic recording medium containing magnetic powders coated with a reaction product between an amino group-containing silane coupling agent and a certain specified compound can have excellent dispersibility of the magnetic powders and excellent durability of painted film and have accomplished this invention.